Coal gasification is the process to convert coal into syngas primarily containing carbon monoxide (CO) and hydrogen (H2). The major types of gasifier are: moving bed gasifier, entrained flow gasifier, and fluidized bed gasifier. The crude syngas leaving the gasifier contains carbon monoxide, hydrogen, water vapor, solids, and hydrocarbons in varying amounts depending on the feedstock material, and type and operating conditions of the gasifier.
In a fluidized bed gasifier, the bed contains a certain amount of small solid particles or “fines”, the amount of which depends primarily on the amount of fines contained in feedstock material and the operating conditions of the gasifier such as the temperature, pressure, velocity and gas/solids contact, etc. A certain portion of the fines is inevitably entrained or carried out by the crude syngas leaving the gasifier. These fines contain unconverted carbon and therefore are a “loss” to the gasifier.
Most fluidized bed gasifiers operate at temperatures somewhat below the ash fusion temperatures to avoid clinker formation in the gasifier which will defluidize the beds and result in involuntary shutdown. The relatively low operating temperatures, and consequently, the non-slagging mode of operation of fluidized bed gasifiers eliminate many design and operating problems commonly associated with slagging entrained flow gasifiers resulting from molten ash at high operating temperatures. The low operating temperatures, however, also limit the carbon conversion of fluidized bed gasifiers. To increase the carbon conversion of fluidized bed gasifiers, the amount of fines loss from the gasifiers must be kept as low as possible, so the fines entrained by the crude syngas must be captured and returned to the gasifiers for more gasification reactions.
Increasing gas and solids residence time in fluidized bed gasifier and/or decreasing the superficial velocity of crude syngas leaving the gasifier will decrease the amount of fines carried over by the crude syngas. However these methods will also significantly increase the costs of the gasifier. A more feasible method to increase the overall carbon conversion of fluidized bed gasifier is to capture and recycle the carried-over fines to the gasifier.
Cyclone systems have been commercially used to capture solids particles entrained by effluent gas from fluidized bed reactors and then return the captured solids to the reactors. However, the solids entrained by the crude syngas from fluidized bed gasifiers often contain very small (e.g. less than 20 μm in diameter) and light solids particles or fines which are difficult to be captured by the conventional cyclones. Furthermore, these fines are often difficult to flow through or discharge from a cyclone dipleg causing frequent solids bridging or plugging problems and ultimately involuntary shutdown of the gasifier and cyclone systems. Different devices and methods using pneumatic and/or mechanical conveyors, valves, eductors, or rotary feeders, for example, have been tried to resolve these problems for fluidized bed reactors operated at high temperatures and pressures. However, the current devices and methods do not work well to provide a reliable long term solution for these problems.
The objective of the present invention therefore resides in providing an apparatus and a method for capturing and recycling small fines for fluidized bed reactors or gasifiers operated at high temperatures and pressures in a more reliable and simpler manner.